Model Based Groundwater Salinization Dynamics and Mechanisms in a Peaty Shallow Aquifer of the Po River Lowland (Italy)

Abstract The Po River (Italy) floodplain is a fragile environment extremely vulnerable to pollution and salinization risk. Here, freshwater availability for agricultural needs has become of concern and farmers are facing crops production decline due to droughts, increasing soil and groundwater salinity. To address salinization problems in complex systems, density dependent numerical flow and transport models are key tools. This study explored the role of stagnant zones, sorption and salt release from peats in modelling salinity dynamics in a coastal shallow aquifer. SEAWAT 4.2 was used to reproduce groundwater heads and salinity dynamics in two adjacent agricultural fields: a cultivated plot where groundwater and soil salinity are not yet reducing crop yields (model A1); and second field plot already unsuitable for cultivation (model A2). Both models were successful in grasping the magnitude and timing of groundwater level and salinity fluctuations over time. Transport models set-up with stagnant zones, sorption and salt release from peats best reproduced the observed trends in comparison to classical advection-dispersion equation simulations that overestimated salinity distribution within both plots. Sorption resulted to be crucial in representing the underlying processes of buffering and delaying salinity pulses, together with additional salt mass accounting for long-term source of salinity within peaty horizons. Mass budget calculations revealed high quantity of salt released from both the domains confirming the low efficiency of subirrigation in freshening these organic-rich peaty environments. This study improves the understanding of key phenomena driving salinization in these fragile lowlands, adding process realism and management relevance.